Bug/deleting docker image result in an integrity error for variant

[aybruhm]

Description

When reserving an app from the CLI, we delete the Docker image associated with the LLM app container to rebuild and assign a new Docker image. During the deletion of the Docker image (app_manager.py line 157 -> update_variant_image function), PostgreSQL raises an IntegrityError exception:

raise translated_error from error
sqlalchemy.exc.IntegrityError: (sqlalchemy.dialects.postgresql.asyncpg.IntegrityError) <class 'asyncpg.exceptions.ForeignKeyViolationError'>: update or delete on table "docker_images" violates foreign key constraint "app_variants_image_id_fkey" on table "app_variants"
DETAIL:  Key (id)=(01908b54-98c0-7528-8604-3653162c7e74) is still referenced from table "app_variants".
[SQL: DELETE FROM docker_images WHERE docker_images.id = $1::UUID]
[parameters: (UUID('01908b54-98c0-7528-8604-3653162c7e74'),)]
(Background on this error at: https://sqlalche.me/e/20/gkpj)

This PR addresses the issue by ensuring the image_id column in the app_variants table allows NULL values. Although we had previously defined an ondelete behaviour, the missing nullable setting caused PostgreSQL to raise the mentioned error above.

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Name	Status	Preview	Comments	Updated (UTC)
agenta	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 7, 2024 4:32am

[aakrem]

thanks @aybruhm